Prior art process instruments, such as Binford U.S. Pat. No. 2,612,572, assigned to the assignee of this application, used floats or the like in conjunction with magnetically operated electrical switches to indicate or control a condition, for example, level. The movement of the float moves a magnet thereby actuating the switch.
It is sometimes necessary to mount the instrument on a process vessel where it may be subject to adverse environmental conditions. A typical example of such an application is in a chemical plant. In certain parts of the plant the air may be dirty or dusty. Also, the operation of the equipment and reactions taking place may produce high temperatures on the order of 750.degree. Fahrenheit. Many process instruments will not operate reliably in this type of environment.
For maintenance reasons, it is also beneficial to have an instrument that can be easily adjusted or replaced with minimal difficulty and without affecting the associated process. Such an instrument could also be retro-fit to existing process vessels.
Prior art process instruments used in the aforementioned environments frequently have a sealed switch, such as a reed switch, to control and indicate a process variable. However, reed switches can only switch circuits operating at low current levels and must therefore be coupled to an interposing relay in order to switch devices requiring higher levels of current. Further, other components in these prior art devices break down at high temperatures.
In view of the above, it is desirable to have a process instrument, such as an electrical switch, that is operable in harsh environments and at high temperatures while switching higher current levels. The present invention is intended to overcome one or more of the problems as set forth above and satisfy these needs.